The present invention relates to a shock absorber with high dissipating power, and more particularly to a shock absorber of the type comprising a rod-and-piston assembly sliding in a cylinder and defining on opposite sides of the piston respective working chambers containing hydraulic fluid, said rod-and-piston assembly being connected to an external source of disturbance and said cylinder being connected to a structure to be protected.
In conventional shock absorbers, a system comprising a telescopic rod and a return spring is used interposed between the structure to be protected (e.g. the bodywork of a motor vehicle) and the source of external disturbances (e.g. a wheel of the vehicle that is directly in contact with the ground). A piston rod and cylinder unit is then provided which is surrounded by the return spring and which has the function of dissipating the energy of the shocks by making use of the viscous flow of the hydraulic fluid. A relationship exists between the friction force F, the displacement speed {dot over (X)} of the liquid, and its viscosity xcex7 (for a Newtonian liquid): the following equation applies F=Gxc2x7xcex7 ({dot over (X)})n where G is a geometrical factor of the solid-liquid system, and n is a power term which generally lies in the range 1 to 4. Energy is dissipated in conventional shock absorbers by transforming mechanical friction energy in the solid-liquid system into heat which is given off to the outside. The amount of energy dissipated is proportional to the speed of the movement to the power n, i.e. xcex94E=K ({dot over (X)})n. In particular, in the event of displacement being large and at very low speed (X≈0), then practically no energy is dissipated.
Shock absorber characteristics, as represented by variations in force as a function of displacement speed, slope to a greater or lesser extent depending on the structure of the shock absorber, and the person skilled in the art knows how to optimize comfort using conventional motor vehicle shock absorbers by lowering the characteristic of the shock absorber as much as possible. However, this leads to a paradox in that in order to have a high degree of energy dissipation and absorption, it is necessary to have a speed that is high.
Document GB-A-2 300 892 describes a shock absorber in which each working chamber is connected to compliant means, in particular an elastically deformable envelope or a gas spring, or indeed a block of closed-cell foam placed in a working chamber. In all circumstances, elastic deformation is used, so the system is reversible and energy is not dissipated.
To complete the technological background, mention can be also be made of document FR-A-85 116 which describes a suspension with variable flexibility, and document FR-A-2 478 763 which describes a hydraulic type energy dissipater.
The present invention seeks to design a novel type of shock absorber that is capable of providing a very high degree of energy dissipation and absorption, while being structurally lighter in weight and more compact than conventional shock absorbers. It is also desired that this novel type of shock absorber should be capable of operating over a relatively broad band of frequencies, given that a conventional shock absorber presents a frequency band that generally goes up to 6 Hz. If frequency values in the vicinity of 6 Hz are exceeded, then the vehicle runs the risk of flying over irregularities in the ground thus causing the wheels to lose adherence with the road.
According to the invention, this problem is resolved by a shock absorber having high dissipating power, of the type comprising a rod-and-piston assembly slidably received in a cylinder and defining on either side of the piston respective working chambers containing a hydraulic fluid, said rod-and-piston assembly being connected to an external source of disturbance and said cylinder being connected to a structure to be protected, in which:
each working chamber communicates continuously with an associated chamber containing a heterogeneous energy absorption-dissipation structure constituted by at least one capillo-porous matrix and an associated liquid relative to which said matrix is lyophobic (not wettable); and
each working chamber also communicates with a common chamber via an associated valve system, said system including non-return means causing the working chamber concerned to close automatically during compression, and causing said chamber to open during expansion, said common chamber constituting a compensation chamber ensuring continuity of the hydraulic fluid during displacements of the rod-and-piston assembly in the cylinder.
The above-specified concept of a heterogeneous energy dissipating and absorbing structure using a capillo-porous matrix and an associated liquid relative to which said matrix is lyophobic is described in detail in the Applicants"" document WO-A-96/18040. In that very innovative type of heterogeneous structure, a solid capillo-porous matrix is used with open pores and controlled topology, having calibrated passages with variations in section and/or interconnections so as to form labyrinths, and a liquid surrounding the capillo-porous matrix defining a solid/liquid separation surface, the matrix being lyophobic relative to the liquid. The separation surface then varies in a manner that is isothermal and reversible as a function of the external pressure to which the heterogeneous structure is subjected. It is thus possible to define genuine pairs of capillo-porous solid matrix and matching liquid that enable quite astonishing energy absorption or accumulation performance to be obtained (by quasi-reversible isothermal processes) and that enable energy to be dissipated (by irreversible isothermal processes) using only the variation in the separation surface in a manner that is quite surprising. The content of the above-specified document is consequently incorporated in the present application by reference.
Provision can be made for the hydraulic fluid occupying the working chambers to be identical to the liquid of the heterogeneous energy absorption-dissipation structures, or in a variant for each heterogeneous energy absorption-dissipation structure to be confined in a deformable leakproof housing, the hydraulic fluid occupying the working chambers then being a conventional engineering fluid.
In a particular embodiment, the rod-and-piston assembly comprises a rod which is hollow on either side of the piston, each hollow portion defining internally a chamber containing a heterogeneous energy absorption-dissipation structure enclosed in a flexible leakproof envelope.
In a variant embodiment, the rod-and-piston assembly comprises a rod which is solid on either side of the piston, and said shock absorber has chambers containing respective heterogeneous energy absorption-dissipation structures enclosed in flexible leakproof envelopes, which chambers are then placed around the cylinder, inside a common housing.
The rod-and-piston assembly of the shock absorber of the invention can be constituted by two portions having the same outside diameter, or in a variant by two portions having different outside diameters, in which case the larger diameter portion is adjacent to the structure to be protected (e.g. the bodywork of a vehicle), and the smaller diameter portion is adjacent to the external source of disturbance (e.g. the wheel of the vehicle).
Each flexible leakproof envelope can be secured to the bottom of the associated internal chamber of the rod-and-piston assembly or to the inside wall of the common housing, as appropriate, or in a variant it can be freely suspended in an associated lateral housing rigidly secured to the central housing and in communication therewith via an associated window, or indeed in said associated internal chamber.
The capillo-porous matrices can be topologically and geometrically identical on either side of the piston, or in a variant they can be topologically and geometrically different so as to impart a controlled amount of asymmetry, and in each case each matrix can be singly-porous or multiply-porous as a function of the stiffness desired for the shock absorber.
Similarly, the lyophobic liquids can have surface tension characteristics which are identical on either side of the piston, or in a variant different on either side so as to impart a controlled amount of asymmetry.
The common compensation chamber can have a flexible wall so as to present a volume that is variable. In particular, under such circumstances, provision can be made for the flexible wall to surround a central portion of the cylinder so as to define an annular chamber constituting the compensation chamber, or for the flexible-walled common compensation chamber to be arranged inside the piston which is made to be hollow, or indeed for the flexible-walled common compensation chamber to be an annular chamber provided at the end of the common housing. In a variant, provision can be made for the common compensation chamber to have a rigid wall and a bottom that is moving or deformable in association with a resilient member (e.g. a volume of gas, a diaphragm, or a piston biased by a spring).
Preferably, the valve system associated with each working chamber includes a throttle defining a calibrated orifice for passing hydraulic fluid coming from the common compensation chamber. In particular, each throttle is individually adjustable, and in particular can be set to a position such that the maximum value of the hydraulic resistance of said throttle corresponds to the value of the capillary pressure at which the liquid intrudes into the pores of the associated matrix.
In a particular embodiment, the non-return means of the valve system associated with each working chamber comprises a deformable flat collar with two branches capable of closing radial orifices of the cylinder which communicate via respective channels with the common compensation chamber.
In another particular embodiment, the non-return means of the valve system associated with each working chamber comprises moving valve members optionally biased by associated springs. In particular, the moving valve members may be arranged at the ends of a central tube opening out into the hollow piston via associated orifices, the compensation chamber containing a toroidal bellows containing air and surrounding said tube.
In which case, it is advantageous for the moving valve members to represent respective central passages constituting calibrated orifices.